Double-layer three-way catalyst with improved aging stability

ABSTRACT

The present invention relates to a catalyst comprising two layers on an inert catalyst carrier, wherein a layer A lying directly on the catalyst carrier contains at least one platinum group metal and one cerium/zirconium/SE mixed oxide, and a layer B, applied on layer A and in direct contact with the flow of exhaust gas, contains at least one platinum group metal and a cerium/zirconium/SE mixed oxide, wherein SE stands for a rare earth metal other than from cerium, characterized in that the fraction of SE oxide in the cerium/zirconium/SE mixed oxide of layer A is less than the fraction of SE oxide in the cerium/zirconium/SE mixed oxide of layer B.

The present invention relates to a three-way catalyst which is composedof catalytically-active layers arranged on top of each other and whichis suitable for cleaning exhaust gases from combustion engines.

Three-way catalysts are used for cleaning exhaust gases from essentiallystoichiometrically-operated combustion engines. In a stoichiometricoperation, the quantity of air fed to the engine corresponds exactly tothe quantity required for complete fuel combustion. In this case, theair-fuel ratio λ—also known as the air ratio—is exactly 1. Three-waycatalysts at around λ=1 are able to simultaneously convert hydrocarbons,carbon monoxide, and nitrogen oxides to harmless compounds.

In general, platinum group metals are used as catalytically-activematerials—particularly, platinum, palladium, and rhodium—that, forexample, are present on λ-aluminum oxide as support material. Inaddition, three-way catalysts contain oxygen-storing materials, e.g.,cerium/zirconium mixed oxides. In the latter case, cerium oxide, arare-earth metal oxide, constitutes the component that is fundamental tothe oxygen storage. Along with zirconium oxide and cerium oxide, thesematerials may contain additional components, such as rare-earth metaloxides or alkaline earth oxides. Oxygen-storing materials are activatedby applying catalytically-active materials, such as platinum groupmetals, and therefore also serve as support material for the platinumgroup metals.

The components of a three-way catalyst may be present in a singlecoating layer on an inert catalyst support; see, for example,EP1541220A1.

Frequently used, however, are double-layer catalysts, which facilitate aseparation of different catalytic processes and, therefore, enable anoptimal coordination of the catalytic effects in the two layers.Catalysts of the latter type are disclosed, for example, inW095/35152A1, WO2008/000449A2, EP0885650A2, EP1046423A2, EP1726359A1,and EP1974809A1.

EP1974809A1 discloses double-layer, three-way catalysts that containcerium/zirconium mixed oxides in both layers, wherein thecerium/zirconium mixed oxide in the top layer has a higher proportion ofzirconium respectively than that in the bottom layer.

EP 1900416A2 describes double-layer, three-way catalysts that containmixed oxides of cerium, zirconium, and niobium in both layers and,additionally, CeZrYLa aluminum oxide particles in the bottom layer.

EP1726359A1 describes double-layer, three-way catalysts that, in bothlayers, contain cerium/zirconium/lanthanum/neodymium mixed oxides with azirconium content of more than 80 mol %, wherein thecerium/zirconium/lanthanum/neodymium mixed oxide in the top layer mayhave a higher proportion of zirconium respectively than that in thebottom layer.

WO2008/000449A2 also discloses double-layer catalysts that containcerium/zirconium mixed oxides in both layers, and wherein the mixedoxide in the top layer again has a higher proportion of zirconium. Tosome extent, the cerium/zirconium mixed oxides may also be replaced bycerium/zirconium/lanthanum/neodymium mixed oxides orcerium/zirconium/lanthanum/yttrium mixed oxides.

WO2009/012348A1 even describes three-way catalysts wherein only themiddle and the top layers contain oxygen-storing materials.

The constantly increasing demand for a reduction in emissions fromcombustion engines requires the continuous further development ofcatalysts. In Europe, the durability requirements have been increasedwith the legislative stage Euro 5 to 160,000 km. The USA even hasdurability requirements of up to 150,000 miles. Therefore, the agingstability of the catalysts has become even more important. The keycriteria for the activity after aging are, on the one hand, thecatalyst's start-up temperatures for the conversion of the pollutantsand, on the other, its dynamic conversion capacity. The start-uptemperature for a pollutant indicates the temperature from which thispollutant will be converted by more than, for example, 50%. The lowerthese temperatures are, the sooner the pollutants can be converted aftera cold start. At full load, exhaust gas temperatures of up to 1,050° C.can occur directly at the motor output. The better the catalyst'stemperature stability, the closer it can be arranged to the engine. Thisalso improves exhaust gas cleaning after a cold start. When the Euro 6cstage comes into force in September 2017, European emission legislationwill stipulate exhaust gas measurements under actual driving conditions.Depending upon driving conditions, this can mean that the catalyst willbe subjected to much more demanding requirements—particularly withrespect to the dynamic conversion of carbon monoxide and nitrogenoxides. These stringent requirements must be met even after strong agingof the catalyst. For this reason as well, the aging stability ofthree-way catalysts must be further increased.

The catalysts according to the aforementioned prior art have very goodproperties with regard to start-up temperatures and dynamic conversioncapacity after aging. However, the increased legal requirements make itnecessary to search for even better catalysts. For this reason, it wasthe problem of this invention to provide a catalyst that, due to itshigher temperature stability, has even lower start-up temperatures andan improved dynamic conversion capacity after aging, compared to thecatalysts of prior art.

Surprisingly, it was found that this problem can be solved if therare-earth elements present as components of the oxygen-storingmaterials, as well as, potentially, the platinum group metals, aredistributed in a specific way on the two layers of a double-layer,three-way catalyst.

The subject-matter of the present invention is, therefore, a catalystcomprising two layers on an inert catalyst support, wherein

-   -   a layer A contains at least one platinum group metal, as well as        a cerium/zirconium/SE mixed oxide, and    -   a layer B applied to layer A contains at least one platinum        group metal, as well as a cerium/zirconium/SE mixed oxide,

wherein SE stands for a rare-earth metal other than cerium,

characterized in that the proportion of the SE oxide in thecerium/zirconium/SE mixed oxide of layer A is less than the proportionof the SE oxide in the cerium/zirconium/SE mixed oxide of layer B,calculated respectively in wt % and relative to the cerium/zirconium/SEmixed oxide.

Layer A and layer B, independently of each other, contain, inparticular, platinum, palladium, rhodium, or mixtures of at least two ofthese platinum group metals, as platinum group metal.

In embodiments of the present invention, layer A contains platinum,palladium, or platinum and palladium, and layer B contains palladium,rhodium, or palladium and rhodium.

In further embodiments of the present invention, the catalyst accordingto the invention is free of platinum.

In particular, layer A contains palladium, and layer B contains rhodium,or palladium and rhodium.

Cerium/zirconium/SE mixed oxides may serve as support materials for theplatinum group metals in layer A and/or in layer B. Furthermore, inlayer A and/or in layer B, they can be supported wholly or in part onactive aluminum oxide.

Therefore, in embodiments of the present invention, layer A and layer Bcontain active aluminum oxide. It is particularly preferable for theactive aluminum oxide to be stabilized by means of doping—particularly,with lanthanum oxide. Preferred active aluminum oxides contain 1 to 6 wt%—in particular, 3 to 4 wt %—of lanthanum oxide (La₂O₃).

The term, “active aluminum oxide,” is known to the person skilled in theart. In particular, it designates λ-aluminum oxide with a surface of 100to 200 m²/g of active aluminum oxide, has been described many times inthe literature, and is commercially available.

The term, “cerium/zirconium/SE mixed oxide,” within the meaning of thepresent invention excludes physical mixtures of cerium oxide, zirconiumoxide, and SE oxide. In fact, “cerium/zirconium/SE mixed oxides” arecharacterized by a largely homogeneous, three-dimensional crystalstructure, which is ideally free of phases from cerium oxide, zirconiumoxide, or SE oxide. However, depending upon the manufacturing process,completely homogeneous products may occur, which can generally be usedwithout any disadvantage.

Lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide,samarium oxide, and mixtures of one or more of these metal oxides may,for example, be considered as rare-earth metal oxides in thecerium/zirconium/SE mixed oxides. Lanthanum oxide, yttrium oxide,praseodymium oxide, and mixtures of one or more of these metal oxidesare preferred. Particularly preferred are lanthanum oxide and yttriumoxide, and a mixture of lanthanum oxide and yttrium oxide is quiteparticularly preferred.

According to the invention, the proportion of the SE oxide in thecerium/zirconium/SE mixed oxide of layer A is less than the proportionof the SE oxide in the cerium/zirconium/SE mixed oxide of layer B,calculated respectively in wt % and relative to the cerium/zirconium/SEmixed oxide.

The proportion of the SE oxide in layer A is, in particular, 1 to 12 wt%—preferably, 3 to 10 wt %, and even more preferably, 6 to 9 wt%—relative to the cerium/zirconium/SE mixed oxide in each case.

The proportion of the SE oxide in layer B is, in particular, 2 to 25 wt%—preferably, 10 to 20 wt %, and even more preferably, 14 to 18 wt%—relative to the cerium/zirconium/SE mixed oxide in each case.

According to the invention, the ratio of cerium oxide to zirconium oxidein the cerium/zirconium/SE mixed oxides may vary widely. In layer A, itis, for example, 0.1 to 1.0 preferably, from 0.2 to 0.7, and even morepreferably, from 0.3 to 0.5. In layer B, it is, for example, 0.1 to1.0—preferably, from 0.2 to 0.7, and even more preferably, from 0.3 to0.5.

The cerium/zirconium/SE mixed oxides in the present invention do not, inparticular, contain aluminum oxide.

In embodiments of the present invention, one layer or both layerscontain alkaline earth compounds, such as barium oxide or bariumsulfate. Preferred embodiments contain barium sulfate in layer A. Thequantity of barium sulfate amounts, in particular, to 5 to 20 g/L of thevolume of the inert catalyst support.

In further embodiments of the present invention, one or both layersadditionally contain additives, such as rare-earth-based compounds,e.g., lanthanum oxide, and/or binders, e.g., aluminum compounds. Theseadditives are used in quantities that may vary widely and which theperson skilled in the art may determine in a specific case by simplemeans.

One embodiment of the present invention relates to a catalyst comprisingtwo layers on an inert catalyst support, wherein

-   -   a layer A contains palladium, active aluminum oxide, as well as        a cerium/zirconium/lanthanum/yttrium mixed oxide, and    -   a layer B applied to layer A contains rhodium, or palladium and        rhodium, active aluminum oxide, as well as a        cerium/zirconium/lanthanum/yttrium mixed oxide,

characterized in that the proportion of the sum of lanthanum oxide andyttrium oxide in the cerium/zirconium/lanthanum/yttrium mixed oxide oflayer A is less than the proportion of the sum of lanthanum oxide andyttrium oxide in the cerium/zirconium/lanthanum/yttrium mixed oxide oflayer B, calculated respectively in wt % and relative to thecerium/zirconium/lanthanum/yttrium oxide,

In this case, it is preferable for the proportion of the sum oflanthanum oxide and yttrium oxide in thecerium/zirconium/lanthanum/yttrium mixed oxide of layer A to be 6 to 9wt % relative to the cerium/zirconium/lanthanum/yttrium mixed oxide oflayer A, and 14 to 18 wt % in the cerium/zirconium/lanthanum/yttriummixed oxide of layer B relative to thecerium/zirconium/lanthanum/yttrium mixed oxide of layer B, calculated ineach case in wt % and relative to the cerium/zirconium/lanthanum/yttriummixed oxide.

In a further embodiment of the present invention, layer A lies directlyon the inert catalyst support, i.e., there is no additional layer or noundercoat between the inert catalyst support and layer A.

In a further embodiment of the present invention, layer B is in directcontact with the exhaust gas stream, i.e., there is no additional layeror no overcoat on layer B.

In a further embodiment of the present invention, the catalyst accordingto the invention consists of layers A and B on an inert catalystsupport. This means that layer A lies directly on the inert catalystsupport, layer B is in direct contact with the exhaust gas stream, andno other layers are present.

Honeycomb bodies made from ceramic or metal with a volume V, which haveparallel flow channels for the exhaust gases of the combustion engine,are particularly suitable as catalytically-inert catalyst supports. Theymay be either so-called flow-through honeycomb bodies or wall-flowfilters.

According to the invention, the wall areas of the flow canals are coatedwith the two catalyst layers A and B. To coat the catalyst support withlayer A, the solids provided for this layer are suspended in water andcoated with the catalyst supports' coating suspension that is thusobtained. The process is repeated with a coating suspension, in whichthe solids that are provided for layer B are suspended in water.Preferably, both layer A and layer B are coated along the entire lengthof the inert catalyst support. This means that layer B completely coverslayer A, and, as a result, only layer B comes into direct contact withthe exhaust gas stream.

In the following examples 1 to 3, and in comparative example 1,double-layer catalysts were produced by twice coating flow-throughhoneycomb bodies made from ceramic with 93 cells per cm² and with a wallthickness of 0.09 mm, as well as dimensions of 11.8 cm in diameter and10.5 cm in length. To this end, two different suspensions were producedrespectively for layers A and B. The support was then first coated withthe suspension for layer A and then calcined in air for 4 hours at 500°C. Subsequently, the support coated with layer A was coated with thesuspension for layer B and then calcined under the same conditions asfor layer A.

EXAMPLE 1

A double-layer catalyst was produced by first producing two suspensions.The composition of the first suspension for layer A (relative to thevolume of the catalyst support) was:

40 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

40 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 25 wt %CeO₂, 67.5 wt % ZrO₂; 3.5 wt % La₂O₃, and 4 wt % Y₂O₃

5 g/L of BaSO₄

3.178 g/L of Pd

The composition of the second suspension for layer B (relative to thevolume of the catalyst support) was:

60 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

47 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 24 wt %CeO₂, 60 wt % ZrO₂, 3.5 wt % La₂O₃, and 12.5 wt % Y₂O₃

0.177 g/L of Pd

0.177 g/L of Rh

EXAMPLE 2

A double-layer catalyst was produced analogously to example 1. Thecomposition of the first suspension for layer A was:

40 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

40 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 20.5 wt %CeO₂, 67.5 wt % ZrO₂, 4.5 wt % La₂O₃, and 7.5 wt % Y₂O₃

5 g/L of BaSO₄

3.178 g/L of Pd

The composition of the second suspension for layer B was:

60 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

47 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 20 wt %CeO₂, 60 wt % ZrO₂, 5 wt % La₂O₃ and 15 wt % Y₂O₃

0.177 g/L of Pd

0.177 g/L of Rh

EXAMPLE 3

A double-layer catalyst was produced analogously to example 1. Thecomposition of the first suspension for layer A was:

40 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

40 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 20.5 wt %CeO₂, 67.5 wt % ZrO₂, 4.5 wt % La₂O₃, and 7.5 wt % Y₂O₃

5 g/L of BaSO₄

3.178 g/L of Pd

The composition of the second suspension for layer B was:

60 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

47 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 15 wt %CeO₂, 60 wt %

ZrO₂, 7 wt % La₂O₃, and 18 wt % Y₂O₃

0.177 g/L of Pd

0.177 g/L of Rh

COMPARATIVE EXAMPLE 1

A double-layer catalyst was produced analogously to example 1. Thecomposition of the first suspension for layer A was:

40 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

40 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 25 wt %CeO₂, 67.5 wt %

ZrO₂, 3.5 wt % La₂O₃, and 4 wt % Y₂O₃

5 g/L of BaSO₄

3.178 g/L of Pd

The composition of the second suspension for layer B was:

60 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

47 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 25 wt %CeO₂, 67.5 wt %

ZrO₂, 3.5 wt % La₂O₃, and 4 wt % Y₂O₃

0.177 g/L of Pd

0.177 g/L of Rh

Example 1 and comparative example 1 were aged in an engine test benchaging process. The aging process consists of an overrun fuel cut-offaging with an exhaust gas temperature of 950° C. in front of thecatalyst inlet (1,030° C. maximum bed temperature). The aging time was76 hours.

The start-up performance was tested on an engine test bench at aconstant average air/fuel ratio λ, and the dynamic conversion was testedwith changes of A. Table 1 contains the temperatures T₅₀ at which 50%respectively of the considered components are converted. In so doing,the start-up performance was determined with a stoichiometric exhaustgas composition (λ=0.999 with ±3.4% amplitude).

TABLE 1 Results of the start-up performance after aging for example 1and comparative example 1 T₅₀ HC T₅₀ CO T₅₀ NOx stoichiometricstoichiometric stoichiometric Comparative 391 402 398 example 1 Example1 381 391 388

The dynamic conversion performance was determined in a range for λ of0.99 to 1.01 at a constant temperature of 510° C. In so doing, theamplitude of λ was ±3.4%. Table 2 contains the conversion at the pointof intersection of the CO and NOx conversion curves, as well as theassociated HC conversion.

TABLE 2 Results of the dynamic conversion performance after aging forexample 1 and comparative example 1 CO/NOx conversion at the HCconversion at λ of the point of intersection CO/NOx point ofintersection Comparative 73.5% 92 example 1 Example 1  79% 93

Example 1 according to the invention shows a significant improvement inthe start-up performance and in the dynamic CO/NOx conversion afteraging.

In the following examples 4 and 5, and in comparative example 2,double-layer catalysts were produced by twice coating flow-throughhoneycomb bodies made from ceramic with 93 cells per cm² and with a wallthickness of 0.1 mm, as well as dimensions of 10.2 cm in diameter and15.2 cm in length, To this end, two different suspensions were producedrespectively for layer A and B. The support was then first coated withthe suspension for layer A and then calcined in air for 4 hours at 500°C. Subsequently, the support coated with layer A was coated with thesuspension for layer B and then calcined under the same conditions asfor layer A.

EXAMPLE 4

A double-layer catalyst was produced by first producing two suspensions.The composition of the first suspension for layer A (relative to thevolume of the catalyst support) was:

70 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

50 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 39 wt %CeO₂, 51 wt %

ZrO₂, 3 wt % La₂O₃, and 7 wt % Y₂O₃

5 g/L of BaSO₄

1.483 g/L of Pd

The composition of the second suspension for layer B (relative to thevolume of the catalyst support) was:

70 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

65 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 24 wt %CeO₂, 60 wt %

ZrO₂, 3.5 wt % La₂O₃, and 12.5 wt % Y₂O₃

0.177 g/L of Pd

0.177 g/L of Rh

EXAMPLE 5

A double-layer catalyst was produced analogously to example 4. Thecomposition of the first suspension for layer A was:

70 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

50 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 25 wt %CeO₂, 67.5 wt %

ZrO₂, 3.5 wt % La₂O₃, and 4 wt % Y₂O₃

5 g/L of BaSO₄

1.483 g/L of Pd

The composition of the second suspension for layer B was:

70 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

65 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 24 wt %CeO₂, 60 wt %

ZrO₂, 3.5 wt % La₂O₃, and 12.5 wt % Y₂O₃

0.177 g/L of Pd

0.177 g/L of Rh

COMPARATIVE EXAMPLE 2

A double-layer catalyst was produced analogously to example 4. Thecomposition of the first suspension for layer A was:

70 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

50 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 39 wt %CeO₂, 51 wt %

ZrO₂, 3 wt % La₂O₃, and 7 wt % Y₂O₃

5 g/L of BaSO₄

1.483 g/L of Pd

The composition of the second suspension for layer B was:

70 g/L of activated aluminum oxide stabilized with 4 wt % of La₂O₃

65 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 22 wt %CeO₂, 68 wt % ZrO₂, 2 wt % La₂O₃, 5 wt % Nd₂O₃, and 3 wt % Y₂O₃

0.177 g/L of Pd

0.177 g/L of Rh

Examples 4 and 5, as well as comparative example 2, were aged in anengine test bench aging process. The aging process consists of anoverrun fuel cut-off aging with an exhaust gas temperature of 950° C. infront of the catalyst inlet (1,030° C. maximum bed temperature). Theaging time was 76 hours.

The start-up performance was tested on an engine test bench at aconstant average air/fuel ratio λ, and the dynamic conversion was testedwith changes in A.

Table 3 contains the temperatures T50 at which 50% respectively of theconsidered components are converted. In so doing, the start-upperformance was determined with stoichiometric exhaust gas composition(λ=0.999 with ±3.4% amplitude) and with slightly lean exhaust gascomposition (λ=1.05 without amplitude).

TABLE 3 Results of the start-up performance after aging for examples 4and 5 and comparative example 2 T50 HC T50 CO T50 NOx stoichio-stoichio- stoichio- T50 HC T50 CO metric metric metric lean leanComparative 403 420 416 383 382 example 2 Example 4 391 411 401 371 369Example 5 384 397 392 370 369

The dynamic conversion performance was determined in a range for λ of0.99 to 1.01 at a constant temperature of 510° C. In so doing, theamplitude of λ was ±3.4%. Table 4 contains the conversion at the pointof intersection of the CO and NOx conversion curves, as well as theassociated HC conversion.

TABLE 4 Results of the dynamic conversion performance after aging forexamples 4 and 5 and comparative example 2 CO/NOx point HC conversion atλ of the of intersection CO/NOx point of intersection Comparative 81.5% 95% example 2 Example 4 86.5% 95.5% Example 5  95% 96.5%

Examples 4 and 5 according to the invention show a significantimprovement in the startup performance and in the dynamic CO/NOxconversion after aging, wherein example 5 shows the greatest activity.

Further examples were prepared analogously to example 5, with thedifference being that rare-earth metal oxides (SE_(x)O_(y)), asspecified in Table 5, were used in the cerium/zirconium/rare-earth metalmixed oxides,

TABLE 5 wt % wt % SE_(x)O_(y) 1 SE_(x)O_(y) 2 Example Layer of CeO₂ ofZrO₂ wt % wt % 6 A 40 50 La₂O₃ 5 — — B 30 55 La₂O₃ 12 — — 7 A 40 50 Y₂O₃7.5 — — B 30 55 Y₂O₃ 15 — — 8 A 40 50 La₂O₃ 5 Pr₆O₁₁ 5 B 30 55 La₂O₃ 5Pr₆O₁₁ 10 9 A 30 63 La₂O₃ 2 Nd₂O₃ 5 B 25 60 La₂O₃ 5 Nd₂O₃ 10 10 A 30 62Nd₂O₃ 3 Pr₆O₁₁ 5 B 30 57 Nd₂O₃ 5 Pr₆O₁₁ 8 11 A 40 54 La₂O₃ 3 Sm₂O₃ 3 B30 55 La₂O₃ 5 Sm₂O₃ 10 12 A 40 51.5 Nd₂O₃ 3.5 Y₂O₃ 5 B 30 55 Nd₂O₃ 5Y₂O₃ 10

The invention claimed is:
 1. Catalyst comprising two layers on an inertcatalyst support, wherein a layer A contains at least one platinum groupmetal, and a cerium/zirconium/SE mixed oxide, and a layer B applied tolayer A contains at least one platinum group metal, and acerium/zirconium/SE mixed oxide, wherein SE stands for a rare-earthmetal other than cerium, wherein the proportion of the SE oxide in thecerium/zirconium/SE mixed oxide of layer A is less than the proportionof the SE oxide in the cerium/zirconium/SE mixed oxide of layer B,calculated respectively in wt % and relative to the cerium/zirconium/SEmixed oxide.
 2. Catalyst according to claim 1, wherein layer A and layerB, independently of each other, contain, as platinum group metal,platinum, palladium, rhodium, or mixtures of at least two of theseplatinum group metals.
 3. Catalyst according to claim 1, wherein, asplatinum group metal, layer A contains platinum, palladium, or platinumand palladium, and layer B contains palladium, rhodium, or palladium andrhodium.
 4. Catalyst according to claim 1, wherein, as platinum groupmetal, layer A contains palladium, and layer B contains rhodium, orpalladium and rhodium.
 5. Catalyst according to claim 1, wherein layer Aand layer B contain active aluminum oxide.
 6. Catalyst according toclaim 5, wherein the platinum group metal in layer A and/or in layer Bis supported wholly or in part on active aluminum oxide.
 7. Catalystaccording to claim 1, wherein the SE oxide in the cerium/zirconium/SEmixed oxide is lanthanum oxide, yttrium oxide, praseodymium oxide,neodymium oxide, samarium oxide, or mixtures of one or more of thesemetal oxides.
 8. Catalyst according to claim 1, wherein the SE oxide inthe cerium/zirconium/SE mixed oxide is a mixture of lanthanum oxide andyttrium oxide.
 9. Catalyst according to claim 1, wherein the proportionof the SE oxide in the cerium/zirconium/SE mixed oxide in layer A is 1to 12 wt % relative to the cerium/zirconium/SE mixed oxide in each case.10. Catalyst according to claim 1, wherein the proportion of the SEoxide in the cerium/zirconium/SE mixed oxide in layer B is 2 to 25 wt %relative to the cerium/zirconium/SE mixed oxide in each case. 11.Catalyst according to claim 1, wherein the weight ratio of cerium oxideto zirconium oxide in the cerium/zirconium/SE mixed oxide in layer A is0.2 to 0.7.
 12. Catalyst according to claim 1, wherein the weight ratioof cerium oxide to zirconium oxide in the cerium/zirconium/SE mixedoxide in layer B is 0.2 to 0.7.
 13. Catalyst according to claim 1,wherein layer A contains palladium, active aluminum oxide, and acerium/zirconium/lanthanum/yttrium mixed oxide, and layer B applied tolayer A contains rhodium, or palladium and rhodium, active aluminumoxide, and a cerium/zirconium/lanthanum/yttrium mixed oxide, wherein theproportion of the sum of lanthanum oxide and yttrium oxide in thecerium/zirconium/lanthanum/yttrium mixed oxide of layer A is less thanthe proportion of the sum of lanthanum oxide and yttrium oxide in thecerium/zirconium/lanthanum/yttrium mixed oxide of layer B, calculatedrespectively in wt % and relative to thecerium/zirconium/lanthanum/yttrium oxide.
 14. Catalyst according toclaim 13, wherein the proportion of the sum of lanthanum oxide andyttrium oxide in the cerium/zirconium/lanthanum/yttrium mixed oxide oflayer A is 6 to 9 wt % relative to thecerium/zirconium/lanthanum/yttrium mixed oxide of layer A, and 14 to 18wt % in the cerium/zirconium/lanthanum/yttrium mixed oxide of layer Brelative to the cerium/zirconium/lanthanum/yttrium mixed oxide of layerB, calculated in each case in wt % and relative to thecerium/zirconium/lanthanum/yttrium mixed oxide.
 15. Catalyst accordingto claim 1, wherein layer A lies directly on the inert catalyst support.16. Catalyst according to claim 1, wherein the proportion of the SEoxide in the cerium/zirconium/SE mixed oxide in layer A is 3 to 10 wt %relative to the cerium/zirconium/SE mixed oxide in each case. 17.Catalyst according to claim 1, wherein the proportion of the SE oxide inthe cerium/zirconium/SE mixed oxide in layer A is 6 to 9 wt % relativeto the cerium/zirconium/SE mixed oxide in each case.
 18. Catalystaccording to claim 1, wherein the proportion of the SE oxide in thecerium/zirconium/SE mixed oxide in layer B is 10 to 20 wt % relative tothe cerium/zirconium/SE mixed oxide in each case.
 19. Catalyst accordingto claim 1, wherein the proportion of the SE oxide in thecerium/zirconium/SE mixed oxide in layer B is 14 to 18 wt % relative tothe cerium/zirconium/SE mixed oxide in each case.
 20. Catalyst accordingto claim 1, wherein the weight ratio of cerium oxide to zirconium oxidein the cerium/zirconium/SE mixed oxide in layer A is 0.3 to 0.5. 21.Catalyst according to claim 1, wherein the weight ratio of cerium oxideto zirconium oxide in the cerium/zirconium/SE mixed oxide in layer B is0.3 to 0.5.